Modern composite materials such as graphite fibers in an epoxy resin matrix, when properly formed, can provide lightweight, high strength and high modulus structures and have a great number of applications. For example, engine nacelles and wings of aircraft and, in fact, entire airframes are now being manufactured using such materials. Thermosetting resins, such as the epoxies, require heating in order to cure and harden. Because such materials are provided in the form of tape or sheet in an uncured state, most structures are formed from multiple layers of this material, often with the fibers in each layer orientated at different angles in order to enhance structural properties. This requires that pressure also be applied during the curing to provide compaction thereof. Typically, flat composite structures can be formed in matched molds or dies; however, a more common approach is to layup the uncured sheets in a mold, vacuum bag, pull a vacuum between the vacuum bag and the mold and thereafter heat and allow atmospheric pressure to compact the layup. If more than atmospheric pressure is required the vacuum bagged layup can be placed in autoclave. In some applications the need for the autoclave can be eliminated by placing the vacuum bagged layup and mold in a housing and use an inflated bag to apply pressure, the limitation being the strength of the bag wall. An example of this technique can be found in U.S. Pat. No. 2,435,866 "Fluid-Operated Apparatus for Producing Molded Articles" by P.H. Bilhuber. However, if the part is large, the mold must be extremely ridged in order to prevent distortion thereto. Thus, mold fabrication costs rise along with the size of the mold and great difficulty is encountered when handling. Another approach is place expandable silicon rubber sheets on one or both sides of the layup. Here the rubber expands upon heating to forming temperatures and supplies the compaction pressure. However, the compaction pressures must be reacted against the molds which also leads to the need for heavy and expensive molds if the parts are large.
In the fabrication of hollow (closed sectioned) composite parts there are primarily two methods used, both based on the above processes for flat type parts. In one method the part is layed up over a rigid mandrel, vacuum bagged and placed in an autoclave. However, if the part is 15 or 20 feet in diameter a very large autoclave is required. Thus, in many instances compaction pressure on the layup is limited to atmospheric and the part must be cured in an oven. Also, the mandrel must be extremely rigid (and heavy) in order to absorb the large compression loads without excessive distortion. The forming pressure can be increased by surrounding the layup with pressurizable bags retained within a housing. However, the addition of the housing makes such tooling even more difficult to handle and costly to manufacture. The same problem occurs if the layup were formed against an external mold.
A specific example can be found in U.S. Pat. No. 4,126,659 "Method of Making a Hollow Article" by L. H. Blad. In this reference a silicon bag in the shape of the inner surface of the structure to be formed is filled with a low melting temperature alloy and allowed to solidify. The part is layed up on the now rigid mandrel with a thin impermeable film therebetween. An external mold is placed about the layup and the film is sealed thereto and a vacuum is drawn. Next the mandrel is heated melting the alloy which is thereafter poured out of the silicon bag. The mandrel is then pressurized and the assembly is heated to form the layup against the external mold. It is obvious that this method would be extremely difficult to use in the manufacture of large diameter parts, especially, when one contemplates filling a 15' diameter bag with the low temperature melting point alloy.
Another approach is found in U.S. Pat. No. 3,962,393 "Method for Making a Hollow Laminated Article" also by L.H. Blad. Here a hollow, flexible envelope is rigidized by means of a solid support mandrel installed therein. As in the prior example, an impermeable film is positioned between the mandrel and the support. A split external mold is placed around the assembly and the film is sealed thereto. A vacuum is pulled and the solid support is removed. The assembly is then placed in an autoclave and heated under pressure compressing the layup against the external mold. The problem here is the difficulty that would be encountered in removing a 15' diameter solid support. Additionally, in such a system, the external mold must be extremely rigid to hold shape. Other patents of interest are U.S. Pat. No. 4,370,120 "Compacting Press with Expandable Body" by D. Foster which uses an expandable circular shaped bag to mold material about a mandrel.
Also of interest is the system disclosed in the U.S. Pat. No. 3,158,089 "Device for Pressing Gruyere Cheeses" by R. Fillol. While this invention relates to an entirely different subject, the molding of cheese, it is of interest because of the duel pressurized bag concept. In this system the cheese is placed in a housing between moveable flat molds. Flexible bags mounted behind each mold are pressurized forcing the molds into the cheese. However, this system could not be used to manufacture composite structural parts because of its inability to hold precise dimensions.
Therefore, it a primary object of the subject invention to provide an apparatus for forming large closed sectioned molded articles made of composite materials and the like.
It is another object of the subject invention to provide an apparatus for making large molded articles without the need for an autoclave while using forming pressures above atmospheric.
It is a further object of the subject invention to provide an apparatus for forming large molded articles using relatively inexpensive tooling and molds.
It is a still further object of the subject invention to provide an apparatus for forming large molded articles which is light in weight.